This study will investigate whether sequential single photon emission computerized tomography (SPECT) imaging with [123I]^D-CIT (2^D-carbomethoxy-3^D-(4- iodophenyl)tropane), a marker of dopamine terminal integrity, will provide a quantitative biomarker of Parkinson's disease progression. This work expands the interface between basic neuroscience and clinical neurobiology of the dopamine transporter to development of a biomarker for Parkinson's disease which has dramatic implications for experimental therapeutics. Previous data has shown that SPECT imaging with [123I]^D- CIT demonstrates a marked reduction in dopamine striatal transporters in patients with Parkinson s disease and that the relative reduction in [123I]^D-CIT striatal uptake correlated with the severity of disease. This study will assess [123I]^D-CIT uptake in Parkinson's disease, for the first time, with a longitudinal study design to simultaneously provide insight into the pathologic mechanism and the clinical progression of disease. Eligible patients will undergo [123I]^D-CIT SPECT imaging and comprehensive neurological evaluation at two years after their initial evaluation. The reduction in [123I]^D- CIT uptake in Parkinson's disease patients will be compared to the reduction in dopamine transporter density in normal aging in a group of healthy subjects. In vivo imaging using SPECT-CIT would be the first widely available, sensitive measure of mesencephalic dopamine neuron loss. This potential biomarker may be useful in diagnosis and prognosis, and to assess clinical progression and possible neuroprotective agents and restorative therapies for Parkinson's disease.